Update a task

Partial update. Send null to clear a nullable field. Moving a task into a section whose name implies a status (e.g. "Done", "In progress", "Blocked") applies that status; completing a task stamps completed_at, moves it to the project's done column (the section named "Done" if present, else the first section whose name implies completed, e.g. "Finished" or "Shipped"), unblocks dependents and notifies watchers. Moving a completed task out of the done column reopens it like POST /tasks/{id}/reopen: status becomes blocked if any dependency is incomplete, else the section-implied status; an explicit status in the same request overrides this.